Design Guidelines to Elongate Spin–Lattice Relaxation Times of Porphyrins with Large Triplet Electron Polarization
Akio Yamauchi, Saiya Fujiwara, Koki Nishimura, Yōichi Sasaki, Kenichiro Tateishi, Т. Уесака, Nobuo Kimizuka, Nobuhiro Yanai
Abstract
The spin-polarized triplet state generated by light irradiation has potential for applications such as triplet dynamic nuclear polarization (triplet-DNP). Recently, we have reported free-base porphyrins as versatile and biocompatible polarizing agents for triplet-DNP. However, the electron polarization of free-base porphyrins is not very high, and the dilemma is that the high polarization of metalloporphyrins is accompanied by a too short spin–lattice relaxation time to be used for triplet-DNP. We report here that the introduction of electron-withdrawing fluorine groups into Zn porphyrins enables a long enough spin–lattice relaxation time (>1 μs) while maintaining a high polarization (Px:Py:Pz = 0:0:1.0) at room temperature. Interestingly, the spin–lattice relaxation time of Zn porphyrin becomes much longer by introducing fluorine substituents, whereas the spin–lattice relaxation time of free-base porphyrin becomes shorter by the fluorine substitution. Theoretical calculations suggest that this is because the introduction of the electron-withdrawing fluorine substituents reduces the spin density on Zn atoms and weakens the spin–orbit interaction.